Cell C incorrect Charges

I upgrade my contract before the time and it was explained to me before I can do it I will have to buy out the contract “pay up in full” for the device that I received and thereafter they will decrease my amount of months remaining in order to upgrade. So I done that. Went to Nedbank payed in the amount the advisor gave me telephonically and 3 days later I went to get the device. I received my bill on the 6th September and my bill came to R1911 and I couldn’t understand why. I never added any extras to my account and I cannot call from my Celephone nor the other contacts I have because I have been listed in the IVR system. So basically I need my bill to be sorted out ASAP. My debit order dare is the 26th of every month. The money will be debited tomorrow which is the 26th but I will have the payment reversed due to Cell C adding the payment that I done at Nedbank to upgrade my contract allocated again to my account so technically I’m payment for it twice. I had a problem 2 months ago which took a whole month to resolve hoping that this one won’t take as long this time around.